This week we look at the claim by Will Happer and Steven Koonin, in Martin Durkin’s Climate the Movie, that warming is not happening uniformly on the thermometer: both say it is concentrated in cold readings rather than warm ones. Happer’s claim was: “And that’s certainly being observed all over the world, if you look at temperature records the high temperatures are almost unchanged. But cold temperatures at night, or during the winter, are going up a little bit. Not very much, but you can measure it.” And Koonin stated that “When the average goes up it’s really more due to the coldest temperatures getting warmer. So the temperature’s getting milder rather than getting hotter.” Here our official fact check is a ringing: Sort of, but there are exceptions.

Trivially, temperatures vary during the day and across the months of the year. As a result, climate records typically include a daytime high and a nighttime low, which are called Tmax and Tmin respectively, and the daily average is the midpoint of the two. So one question is whether Tmax and Tmin are both warming at the same rate and Happer says no, Tmin is going up but Tmax is changing very little. So is he right?

Up to a point. In the US, according to the 2017 National Climate Assessment, it is true that minimum temperatures have gone up more than maximum ones. Table 6.1 in that report states that comparing the 1986-2016 average to the 1901-1960 average, while Tmin rose 1.41°F, Tmax only rose 1.06°F. So daytime highs are not “almost unchanged” but they are changed by less than nighttime lows. The same report also states (p. 187) that “warming was greatest and most widespread in winter, with increases of over 1.5°F (0.8°C) in most areas.” Oddly they don’t report the summer change but they show pictures of the two seasons (p. 188) as follows:

Interestingly summertime highs have cooled in about half the US. So again the warm season temperatures actually have changed – in the eastern US they’ve gone down.

The situation in Canada is the same. Our 2017 federal climate change report says (p. 125) that between 1948 and 2016 summertime temperatures went up 1.5ºC and wintertime temperatures went up by 3.3ºC, more than twice as much. But, in fairness, summertime temperatures in Canada might be thought of as cold season temperatures in comparison to, say, Mississippi or Texas, so arguably it is another instance of more warming where it was colder to begin with.

For the world as a whole the picture gets rather complicated. In a 2019 paper a group of Chinese scientists reported the following trends for the world and for the Northern and Southern Hemispheres separately:

Over 1901 to 2014 they computed a global trend of 0.10ºC per decade in Tmax and 0.14ºC in Tmin, which fits Koonin and Happer’s claims. And the pattern also holds in the NH and SH separately. But focusing in on the 1951-2014 period in the SH, Tmax went up a bit more than Tmin (0.131ºC versus 0.125ºC per decade). And in the 1998-2014 period also in the SH, while Tmax went up by 0.107 ºC per decade Tmin actually went down by 0.132ºC per decade. Which is opposite to the claimed pattern, not just of Happer and Koonin but of people who say the Earth is warming, period.

So overall we conclude that in general, and in the US specifically, yes, nighttime lows are warming faster than daytime highs, winters are warming more than summers, and summertime highs have barely changed on average (they’re higher in the west and lower in the east). Koonin is right that the climate is becoming more mild and Happer is basically right regarding the overall US averages. But globally, while the pattern holds over the whole 20th century, over the past couple of decades for some reason it didn’t hold in the Southern Hemisphere.

Apparently climate is complicated. Who knew?